Season the chicken pieces with salt and pepper. In a large dutch oven , heat the oil and butter until it sizzles. Add the chicken pieces, fat side down and brown on high heat. Turn the chicken and brown the other side. Remove the chicken pieces and set aside on a plate. Remove excess fat from the pot, leaving about three tablespoons for sauteing the vegetables. Saute the onions ,scraping the chicken bits from the pot; add the garlic and leeks. Continue cooking on medium heat for 3 to 4 minutes minutes. Add the mushrooms; allow to cook for a bout 5 minutes. Gradually mix in the flour and continue stirring while adding the wine. Allow the flavors to infuse and flour to cook, continue cooking for a minute or two. Stir in the chicken broth. Return the chicken pieces to the pot, placing them on top of the mushrooms. Simmer for35 minutes or until the chicken is cooked and tender. Serve with buttered pasta, boiled potatoes, or simply alone with a salad and french bread. Bon apetit!